Highly alloyed martensitic tool steels have been used on a limited basis in automobile engine and drive-line applications in spite of the fact that they exhibit good strength (hardness) and wear resistance at elevated temperatures. The factors which discourage greater use of such tool steels are: (a) they contain substantial amounts (greater than 4-5%) of alloying ingredients, such as chromium, molybdenum, tungsten, cobalt or vanadium; (b) they are expensive and difficult to heat treat for final hardness requiring temperatures of 1000.degree.-1250.degree. C. for austenization; and (c) they are expensive and difficult to soften for fabrication into parts prior to heat treatment.
Instead, more economical low alloy carburizing steels are commonly used in automotive applications; these steels, however, lack sufficient strength and wear resistance at high temperatures for many applications. Such carburizing steels do not exhibit secondary hardening during tempering, but progressively soften as the tempering or service temperature is increased. Thus, for automotive engine parts which are at times imperfectly lubricated, such as engine valve trains, conventional carburizing steels are not entirely satisfactory. Their wear resistance would be better if they exhibited secondary hardening behavior.
Modifications of carburizing steels have been made by the prior art to make them more temperature resistant, but such modifications have involved adding large amounts of alloying ingredients, substantially to the level of tool steels, and thus suffer from the three disadvantages outlined above for tool steels.
The alloying ingredients of molybdenum, tungsten, vanadium and chromium, which are now known as the result of this invention to be effective in producing secondary hardening when used with proper heat treatment and used in economical controlled amounts, have been used by the prior art, but such use has been consistently in improper amounts and with the wrong associated processing so that the combined benefits of this invention are never realized.
In U.S. Pat. No. 2,992,148, a teaching is made for the making of steel designed to have good magnetic as well as mechanical properties; it also is to be used in a normalized condition after heat treatment, thus making it a bainitic steel rather than a martensitic steel. Although the disclosure suggests the use of molybdenum and vanadium as well as chromium, the molybdenum and vanadium additions are too low in amount and are associated with the wrong thermal processing; significant secondary hardening would not be produced by the normalizing heat treatment described. In addition, such disclosure requires a high nickel content; in our invention, the nickel content should not exceed about 0.25%, the maximum level of nickel usually found as a residual impurity in alloy steels.
In U.S. Pat. No. 2,206,370, there is also a teaching of the use of small amounts of molybdenum and vanadium combined with a high nickel content. Little or no secondary hardening can take place with this alloy because the molybdenum and vanadium contents prescribed for use are too low and the final austenitizing temperature (1500.degree.-1550.degree. F.) is too low for dissolution of alloy carbides.
In U.S. Pat. No. 3,257,200, small additions of vanadium and columbium are made to a 21/4% chromium and 1% molybdenum steel to improve creep strength at elevated temperatures without degrading impact properties. The amount of chromium required is too high in conformity with the teaching of this invention because chromium restricts the solubility of carbon in austenite at 950.degree. C. The molybdenum content is too low for optimum secondary hardening response. 21/4 Cr - 1 Mo steel was designed for creep resistance and is commonly used for boiler tubing in steam power plants and for heat exchanger tubing. Because it was formulated for an entirely different purpose, the mechanical properties of 21/4 Cr - 1 Mo steel after carburizing would be distinctly inferior to the properties of the alloys described in this invention.